1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, an electronic device, and the like are all semiconductor devices.
2. Description of the Related Art
A technique for forming a transistor by using a semiconductor film formed over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit (IC) and an image display device (display device). A silicon-based semiconductor is widely known as a semiconductor applicable to the transistor. In recent years, an oxide semiconductor has attracted attention.
For example, a transistor in which an amorphous oxide film containing indium, gallium, and zinc and having an electron carrier concentration of lower than 1018/cm3 is used for a channel region of the transistor is disclosed (see Patent Document 1).
A transistor including an oxide semiconductor film is known to have a problem of low reliability because of possibility of change in electrical characteristics, although the transistor including an oxide semiconductor film can operate at higher speed than a transistor including amorphous silicon and can be manufactured more easily than a transistor including polycrystalline silicon. Specific examples of the problem include change in threshold voltage of the transistor after a bias-temperature test (BT test).